In the Indian tropical region, the seasonal monsoon driven by ocean-land temperature contrasts has a major impact on human life. The monsoon is perhaps the classic example of ocean-atmosphere-land interactions. During boreal summer, a northward shift of the ITCZ to the Indian sub-continent creates a major precipitation and heat source in this region. Interannual variability in the Indian monsoon correlates closely with tropical Indian Ocean SST. Indian Ocean SST is affected by ENSO and by an intrinsic Indian Ocean east-west mode of variability similar in mechanism but uncorrelated with the Pacific’s ENSO.

Extended Summer Drought

The Northern Hemisphere’s annular mode and the decade-to-decade ENSO-like variability discussed in the previous sections both affect Northern Hemisphere climate mainly during the winter season, and they involve the atmosphere’s own preferred modes of month-to-month variability. In contrast, drought and desertification, when they occur in extratropical latitudes, are primarily summer phenomena whose geographic distribution and evolution are determined as much by land-surface processes as by atmospheric dynamics. Dynamical modes may still be involved, however, as the summer pattern of great anticyclones over the oceans responds to the heating of the continents. Kelvin and Rossby waves are active in determining the shape, extent, and flow of moisture in this pattern (Rodwell and Hoskins, 2001), and in turn these waves are involved in dynamical modes as noted above.

An extended drought popularly known as the Dust Bowl affected large areas of the United States through most of the decade of the 1930s. Over parts of the Great Plains and Midwest, the 1931-1939 summers were on the average substantially warmer than the long-term climatological mean for the season, with daily maxima often in excess of 40°C, and precipitation was deficient (Borchert, 1950; Skaggs, 1975; Karl and Quayle, 1981; Diaz, 1983; Chang and Wallace, 1987; Chang and Smith, 2001). Much of the topsoil was irreversibly lost—blown away in dust storms that darkened skies as far downstream as the eastern seaboard. Numerous farms were abandoned, and agricultural productivity dropped sharply. Many who lived through the Dust Bowl must have wondered whether climatic conditions would ever be suitable for farming again. Yet toward the end of the decade, the rains returned, and the region has never since been plagued by such an extended drought.



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